Contact suspension for a bistable actuator

ABSTRACT

In a bistable switch mechanism having a biasing system defining a nonlinear negative spring rate which renders the mechanism responsive to excursions of actuating stimuli outside a selected range of values of such stimuli and which has a substantially lower negative spring rate for one position of the switch than for the other stable position, there is provided a resilient mounting for a movable contact which has structural configuration and spring stiffness arranged relative to the spring characteristics of the biasing system to define an overall negative spring rate for the mechanism which has a higher value of negative spring rate, for the one position of the switch, than the spring rate of the biasing system alone, when the force versus deflection curve of the bias system has an average slope between limits corresponding to said stable positions which is equal to the average slope of the corresponding curve of the entire mechanism between like limits, so that the operating characteristic of the mechanism in either direction becomes more nearly uniform and optimum.

United States Patent [72] Inventor Irven H. Culver Playa Del Rey, Calif. [21] Appl. No. 715,022 [22] Filed Mar. 21, 1968 [45] Patented Apr. 27, 1971 [73] Assignee Southwestern Industries Inc.

Los Augeles, Calif.

[54] CONTACT SUSPENSION FOR A BISTABLE ACTUATOR 19 Claims, 12 Drawing Figs.

[52] US. Cl 200/67, 200/166 [51] Int. Cl. ..HOL 13/24, H01 h 13/28 [50] Field of Search 200/67 (0), 67 (D2), 166 (H); 337/347 [56] References Cited UNITED STATES PATENTS 2,565,350 8/1951 Burns eta] 200/67D(UX) 2,606,259 8/1952 Huetten ..200/l66H(UX) 3,176,109 3/1965 Wodtke ..200/166H(UX) 3,349,201 10/1967 Davis ..200/67D2(UX) 3,395,375 7/1968 Risketal. 337/347X Primary Examiner-Robert K. Schaefer Assistant Examiner- David Smith, Jr. Attorney-Christie, Parker and Hale ABSTRACT: In a bistable switch mechanism having a biasing system defining a nonlinear negative spring rate which renders the mechanism responsive to excursions of actuating stimuli outside a selected range of values of such stimuli and which has a substantially lower negative spring rate for one position of the switch than for the other stable position, there is provided a resilient mounting for a movable contact which has structural configuration and spring stiffness arranged relative to the spring characteristics of the biasing system to define an overall negative spring rate for the mechanism which has a higher value of negative spring rate, for the one position of the switch, than the spring rate of the biasing system alone, when the force versus deflection curve of the bias system has an average slope between limits corresponding to said stable positions which is equal to the average slope of the corresponding curve of the entire mechanism between like limits, so that the operating characteristic of the mechanism in either direction becomes more nearly uniform and optimum.

" Patented Aril27, 1971 V Y 3,576,410

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Pa tented April 27, '1971 5 Sheets-Sheet 3 1 w L? W a 0 7 J 4 w INVENTOR. [new A/ Jm/ie %/Z Jfmn/Erf Patented April 27, 1971 5 Sheets-Sheet .4.

. INVENTOR. Jmq/ fuzz 5e mechanism has undesirable effects on the i cosrrsc'r SUSPENSION son A BlSTABlLlE ACTUATOR BACKGROUND OF THE INVENTION l. Field of the invention This invention relates to bistable electric switches conconnect a source of'pressurizing fluid to the tank when pressure in the tank falls below a selected pressure.

Such pressure sensitive switches often are subjected to exrange extending from 350 F., or lower, to +350 F., or higher, and to withstand shock and vibration loadings of up to 100 Us or more in magnitude.

position and the switch is operated back to its original state.

in view of the environment in which the device must funcnegative spring rate.

it should be understood at this point that the spring rate of a resilient system is the first derivative of the force versus deflection curve of the system, such a curve being obtained by positively, the load resulting from (or productive of) such deflection is decreased in magnitude. Coil springs, for example, show positive spring rates in that greater and greater deflections of the spring result in more and more force being developed by the spring.

Because of the geometry of the biasing mechanism used in linear spring rate. Operation of the device in a highly nonforce/deflection curve of the biasing operation of the switch. in such instances, the rate of change of the biasing force applied to the stem is less in one of the two stable posiin the other. Therefore, the stern, in-

nonlinearity also causes the switch to have characteristics when make or break" between the contacts occurs substantially instantaneously, from one of its stable states to the other.

Prior attempts to eliminate the nonlinearity of the force applied by the biasing mechanism to the stem ful in a practical fail under high shock conditions or ex tremely low temperatures. An increase in the physical size and weight of the device is highly undesirable since increased weight renders the mechanism more sensitive to acceleration and vibration. Also, prior attempts at eliminating spring rate SUMMARY OF THE INVENTION This invention provides simple, efficient, effective and lightweight apparatus for suitably modifying the normally nonlinear negative spring rate of the biasing mechanism of the above-described pressure monitoring switch, for example. The

relative to the forces produced in the basic biasing mechanism, as are required to produce the desired modification of the force/deflection curve of the basic biasing mechanism. The modification of the spring rate results in a device which operates essentially the same in either direction. Also, the desired snap or toggle action of the device is not impaired. with the result that electrical contacts do not chatter or bounce as the device is operated.

It will be understood, however, that the present invention has applications in devices other than in the specific pressure monitoring switch described above.

Generally speaking, this invention is provided in the context of a bistable electric switch mechanism which includes a housing and movable contact means having two stable positions in the housing. Stationary contact means in the housing are engageable with the movable contact means in at least one of the stable positions of the movable contact means. Bistable actuating means are connected to the movable contact means -and are responsive to actuating stimuli having values either greater than a first selected value or less than a second lesser selected value for moving the movable contact means between its stable positions. The actuating means includes biasing .means urging the movable contact means to the one position and against which an actuating stimulus must act to produce movement of the movable contact means from its one to its other position. The biasing means has a nonlinear spring rate the value of which is less when the movable contact means is at its one position than when the movable contact means is at its other position. In this context, the invention comprises resilient means movably mounting the movable contact means to the housing for movement thereof in response to operation of the actuating means. The resilient means has spring rate and configuration selected and arranged in cooperation with the biasing means to define a negative spring rate for the complete mechanism which has a value, when the movable contact means is at its one position, which is greater than the corresponding spring rate value of the biasing means alone, assuming the force versus deflection characteristics of the biasing means and the overall mechanism are plotted graphically between limits corresponding to said two stable positions of the movable contact means so that the two resulting curves have equal average slope.

DESCRIPTION OF THE DRAWINGS The above-mentioned and other features of the invention are more fully set forth in the following detailed description of a presently preferred embodiment of the invention, which description is presented with reference to the accompanying drawings, wherein:

FIG. 1 is a simplified cross-sectional elevation view of a pressure monitoring switch and illustrates the structure of the present invention in schematic form;

FIG. 2 is an enlarged elevation view of the mounting means for the movable contacts of the switch shown in FIG. 1;

FIG. 3 is a perspective view of an essentially complete movable contact mounting means;

FIG. 4 is a perspective view of an initial state of the fabrication of the structure movable contact mounting means shown in FIG. 3;

FIG. 5 is an enlarged cross-sectional view taken along line 5-5 in FIG. 2;

FIG. 6 is a graphical representation of the force versus deflection characteristic of the biasing means of the switch shown in FIG. 1;

FIG. 7 is an enlarged view of a portion of the curve presented in FIG. 6;

FIG. 8 is a graphical representation of the sum of the portion of the curve presented in FIG. 7 and of the corresponding curve describing the spring characteristic of the movable contact mounting means shown in FIG. 2;

FIG. 9'is a representation similar to FIG. 8 depicting the sum of the force versus deflection curves of the movable contact mounting means represented in FIG. 8, and of the biasing means operating in a portion of the curve of FIG. 6 different from that amplified by the representation of FIG. 7;

. FIG. 10 is'a representation of only the summation curve of FIG. 9;

FIG. 11 is a representation similar to that of FIG. 10 depicting a more nearly optimum summation curve for the switch shown in FIG. 1; and

FIG. 12 is a representation similar to that of FIGS. 8 and 9 showing the component curves producing the summation curve of FIG. 11.

DESCRIPTION OF A PREFERRED EMBODIMENT A diaphragm pressure switch 10, shown in simplified form in FIG. 1 as an example of a device in which the invention has utility, includes a housing 11 defining an internal chamber 12 having an upper part 13 and a lower part 14 connected by a neck portion 15. A diaphragm 16 is disposed across chamber part 14 and has its periphery clamped to the housing. The diaphragm divides chamber part 14 into a system chamber 17, to which fluid from a monitored fluid system is applied at system pressure through a port 18, and into a vent chamber 19 which is communicated by a port 20 to ambient pressure conditions. The fluid pressure supplied to system chamber 17 may be the pressure existing at a selected point in a liquid oxygen system in a liquid fueled rocket, for example. The portion of chamber 12 lying above the diaphragm may be hermetically sealed, if desired. The pressure presented to the upper side of the diaphragm is referred to herein as reference pressure.

An operating stem 22 is connected at its lower end to the center of the diaphragm and extends through neck 15 into chamber 13. The upper end of the stem extends through a passage 23 which interconnects chamber 13 with a switch chamber 24. In the switch chamber, the stem is coupled to two movable contact pads 25, 26 of an electrical switch 27 disposed in chamber 24. Contact pad 25 cooperates with a stationary contact 28 and contact pad 26 cooperates with a stationary contact 29 of the switch. Contact pads 28 and 29 are mounted to the ends of a corresponding pair of support arms 30 and 31, the arms being conductive and connected into the controlled circuit by conductors 32. The movable contact pads are connected into the controlled circuit by a conductor 33.

Within chamber part 13 the stem is coupled to a negative spring rate biasing mechanism 34, the basic biasing mechanism of device 10. As shown in FIG. 1, this mechanism may include a Belleville spring 35 disposed concentric to the stem and having its inner rim 36 confined between a pair of keeper collars 37 secured to the stem on opposite sides of the Belleville spring. The outer periphery of the Belleville spring is disposed above the inner rim along the stem and is engaged in a recess 38 formed circumferentially of chamber 13. The Belleville spring is preloaded so that, throughout the range of reciprocal travel permitted to the stem along its length, the spring operates within that portion of its force/deflection curve having a negative slope. That is, spring 35 has a negative spring rate in terms of upward travel of the stem and of loads imposed upwardly upon the spring by the stem.

Stem 22 is confined to reciprocatory motion between limits which are defined by opposite stop surfaces 40 and 41 ofa recess 42 formed in the housing about the stem along neck 15. The stop surfaces cooperate with a stop collar 43 secured to the stem. The effective displacement D afforded to the stem by the stop surfaces and the stop collar corresponds to the deflection range referred to in the subsequent description of device 10. The magnitude of displacement D may be on the order of as little as 0.006 inch.

From the description presented thus far, it is apparent that device 10 is a bistable mechanism responsive to pressure presented to system chamber 17 via port 18. Assume that the device is in the state shown in FIG. 1 with stop collar 43 engaged with bottom stop surface 41. Any increase in pressure bias of spring 35 until the pressure in the system chamber again rises above P Thus, device responds to excursions of system pressure outside the pressure range defined between P,

and P to generate electrical signals used by structure (not shown) to perform desired control functions.

Relative to Belleville spring 35, upward movement of stem 22 is regarded as movement in a positive direction. The negative spring rate of the Belleville spring gives the desired toggle action to the movement of the stem, which toggle action assures that the stem is in one or the other of its two stable positions depending upon the pressure present in system chamber 17.

Referring to FIG. 6, curve 45 is a graphical representation of the force/deflection characteristic of spring 35, the displacement of spring inner rim 36 upwardly being plotted horizontally (this quantity corresponds to upward movement of stem 22) and the force required to produce such deflection of the spring inner rim being plotted vertically (such force corresponds to the force applied downwardly upon the stem by the spring). Curve 45 has a maximum at 46 and a minimum at 47 between which the slope of the curve is negative; through such a deflection range the spring manifests a negative spring rate. As noted above, in device 10 spring 35 is preloaded to operate in its range of negative spring rate through a deflection range corresponding to displacement of the stem through a distance D.

As illustrated in FIG. 1, support arms 30 and 31 for switch contacts 28 and 29 are spaced sufficiently close that the distance between contacts 28 and 29, less the distance normally provided between contacts 25 and 26, is less than distance D times the motion amplification factor of the linkage system of a mounting assembly 44 for contacts 25 and'26 coupled to stem 22 in chamber 24. Thus, when the stem is in either of its stable positions, the one of contacts 25, 26 which is engaged with the appropriate one of contacts 28, 29 is deflected from the position it would otherwise occupy so that the engaged contacts are forcibly engaged with each other to resist separation by vibrations or shocks applied to housing 11. Because of this feature of the electrical switch in device 10, it is apparent that a finite amount of travel of the stem is required to produce a device which will withstand vibrations and shocks of a defined magnitude without generating spurious signals through conductors 32 and 33. If this distance of travel is reduced, the ability of the device to withstand vibration and shock is also reduced. Therefore, parameter D described above is essentially a fixed parameter in any device of the type described capable of meeting specified environmental conditions.

The problems of stem creep and contact bounce were reviewed above. These problems are minimized where, according to the prior art, device 10 is constructed so that the portion of curve 45 corresponding to stem travel through distance D is linear or essentially linear; such a portion of curve 45 is indicated in FIG. 6 by deflection range 48 corresponding to spring deflection limits 49 and 50 and to a force difference of AF, applied to stem 22. Recalling that distance D is essentially fixed for a particular class of device 10, it is apparent that use of spring 35 through range 48 of curve 45 necessarily means that the device must be used to monitor a rather broad pressure range, the force applied to the stem at deflection condition 49 being attributable to pressure P, and the force applied to the stem at deflection condition 50 being attributable to pressure P If the device is to be operated in connection'with a pressure range productive of a force differential applied to the stem of less than AF the variation indicated by the prior art was to shift deflection range D laterally along curve 45, say to the left to be between spring deflection conditions 5i and 52 (defining deflection range 53 of spring 35) as shown in FIG. 6, thereby reducing the force differential associated with deflection range D and also significantly and undesirably subjecting the operation of the device to the disadvantages of a nonlinear negative rate biasing mechanism. The use of a range of curve 45 which bracketed maximum 46 was decidedly not preferred because such expedient sacrificed the desired toggle action of the mechanism and produced a device which tended to hunt in response to changes in pressure applied to diaphragm 16.

This invention enables the use of that portion of curve 45 which more nearly approaches deflection range 53 than deflection range 48 without encountering the problems associated with a nonlinear spring rate in the negative rate biasing mechanism, while providing a device responsive to a small force (pressure) differential AF As shown in schematic form in FIG. 1, mounting assembly 44 for movable switch contacts 25 and 26 includes a switch contact support lever 55 pivotally mounted between its ends at a fulcrum point 56, and a push element 57 hinged at 58 to the end of the contact support lever opposite from the movable contacts and hinged at 59 to the upper end of stem 22. The mounting assembly also includes a stay member 66 coupled between point 59 and the housing to assure that the upper end of the stem moves only reciprocably and not laterally. These elements of the mounting assembly are mounted so that contacts 25 and 26 move downwardly when the stem moves upwardly, and so that the assembly manifests a motion amplification factor (reciprocal of mechanical advantage) substantially greater than unity from the stem to the movable contacts.

The structure of mounting assembly 44 is shown best in FIGS. 2, 3 and 4. The contact support lever (see FIGS. 2 and 4) is an elongate metallic element having depending lateral stiffening flanges 61 and a rear terminal tongue 62 which is inclined downwardly at an angle of about 45 away from the body 63 of the lever at the end thereof opposite from contacts 25 and 26. An aperture 64 is formed through the body to permit the stem to be passed through the lever, the normal undisplaced position of the lever being that shown in FIG. 2 wherein the stern and the lever are substantially perpendicular to each other.

A bridge member 65 is secured to the upper surface of lever 55 between aperture 64 and tongue 62 and extends transversely of the support lever. The length of the bridge member is substantially greater than the width of the support lever across flanges 61 so that the bridge member defines end lugs 66 extending laterally of and normal to the body of the support lever. An elongate aperture 67, aligned with the length of the bridge member, is formed therethrough over the support lever. A right-angled clip 68 is connected across the support lever between the bridge member and aperture 64 to define a pair of lugs 69 which extend parallel to the body of the support lever laterally of the edges of the support lever and forwardly of the bridge member relative to tongue 62. The clip also serves to maintain the bridge member rigid relative to the support lever.

The contact support lever is mounted to housing 11 by two sets of perpendicularly crossed leaf springs 70 and 7 l, the line of intersection of the planes of the leaf springs defining fulcrum axis 56. Springs 70 are disposed in a common plane lying parallel to stem 22 on the side of the stem opposite from switch 27. The lower ends of springs 70 are secured between opposing vertical flanges 72 of a pair of mounting brackets 73 which have horizontal bodies 74. The mounting brackets are secured to housing 11 by screws 75, the brackets being maintained out of conductive contact with the housing by insulative washers 76 engaged with the opposite sides of the bracket bodies around the screws.

The upper ends of springs 70 are secured to the outer ends of the terminal lugs of bridge member 65, as shown in FIG. 3. Backup plates 77 may be used to facilitate the connection of the springs to the bridge member by rivets 78 or by spot welding.

Springs 71 are defined as parallel legs of a generally l-I- shaped spring sheet which is disposed between a pair of similarly configured backup plates 79, the legs of the spring sheet defining springs 71 extending beyond the ends of the corresponding legs of the backup plates. Backup plates 79 are clamped between a pair of insulator blocks 80 which are secured to the housing rearwardly of stem 22 from switch 27 by screws 81. Screws 81 are maintained out of contact with the backup plates, and the spring sheet therebetween, by insulative grommets 82 (FIG. 2) disposed around the screw shanks within apertures 83 formed through the backup plates and the spring sheet. One of the rear legs of one of backup plates 79 extends rearwardly of the insulator blocks to serve as a connection lug 84 for conductor 33 associated with movable switch contacts 25 and 26.

Springs 71 pass parallel to the body of support lever 55, between springs 70 and the support lever, into engagement with clip lugs 69 to which the springs are secured by backup plates 77 and rivets 78.

Springs 70 and 71 cooperate to define a flexure which constrains the support lever from all motion relative to the housing except pivotal motion about the line defined by the intersection of the planes of springs 70 and 71.

Push element 57 is defined by an elongate piece of thin spring metal which has one end thereof connected to terminal tongue 62 of the support lever by a backup plate 77 and rivets 78. From the tongue, the push element extends upwardly between springs 71 and through bridge member aperture 67 to its other end which is secured between a pair of clamp plates 86 defining coaxial apertures 87 (FIG. 3) through which the stern passes. The push element is arranged to be loaded essentially in tension or compression during operation of switch 10. Clamp plates 86 are secured to stem 22, but are maintained out of conductive contact with the stem by a pair of insulator washers 88 between which the clamp plates are held. Washers 88 are held in place along the stem by nuts 89 threaded to the upper end of the stem in switch chamber 24.

Stay member 60 is provided as a length of thin spring metal and has one end clamped between upper nut 89 and upper washer 88. The other end of the stay member is held to the upper surface of upper insulator block 80 by the heads of screws 81.

Movable switch contact pad 25 is mounted to the free end of a support spring leaf 90 which is mounted in cantilever fashion to the forward end of support lever 55, as shown in FIGS. 2, 3 and 4. A pick member 91 is associated with contact 25. The pick member, between the end of lever 55 and contact 25, has a C-shaped configuration. The end of the pick member remote from the lever normally engages leaf 90 adjacent contact 25.

Movable contact 26 is mounted to the free end ofa support spring leaf 92 which is mounted in cantilever fashion to the forward end of the support lever parallel to leaf 90. A pick member 93, similar in configuration to pick member 91, is associated with leaf 92 and normally engages the leaf adjacent contact 26. Leaf 92 is made of somewhat heavier gauge spring metal than is leaf 90.

The contact support leaves and the pick members are arranged so that leaf 90 moves downwardly away from engagement with pick member 91 and leaf 92 moves upwardly out of engagement with pick member 93.

As shown in FIG. 5, a stationary contact 28 is mounted above contact 25 and stationary contact 29 is mounted below contact 26.

Assume that the structure of switch were not as described above, but were such that contacts and 26 were mounted on a resilient arm secured in cantilever fashion directly to stem 22; such different structure approximates prior devices. Assume further that device 10 were operated through that deflection range of biasing mechanism 34 represented by range 53 in FIG. 6 to be responsive to a pressure induced force differential AF, applied to stem 22 by diaphragm 16. If such were the case, FIG. 7 (being an enlargement of the portion of curve 45 through deflection range 53, and in which the notation "TS" denotes top stop stern position and the notation BS" denotes bottom stop stem position) illustrates the energies available in the switch mechanism during operation of switch 10 in either direction. Shaded area 95 represents the energy expended in movement of the stem from its bottom stop (engagement of collar 43 with surface 41) to its top stop (engagement of collar 43 with surface 40). The total area of area 95 from bottom stop limit 51 to any given point along curve 45 between limits 51 and 52 represents the momentum of the stern, and of the contact carried by it, at the given point. Similarly, shaded area 96 in FIG. 7 represents the energy expended in movement of the stem from its top stop to its bottom stop, such area also providing information concerning the momentum possessed by the stem and the contact carried thereby during top to bottom movement of the stem.

If the conditions assumed in the preceding paragraph exist, FIG. 7 shows that, through range 53, the stem tends to creep off its bottom stop in moving from its bottom to its top stop. This is true because the energy required to produce such movement is expended slowly during the initial stages of such movement. In moving from its top to its bottom stop, however, the stem begins to move rapidly since the energy required to produce such movement is expended rapidly during the early stages of such movement. The result is that a switch contact carried by the stem separates slowly from its cooperating stationary contact as the stern begins to move from its top to its bottom stop, and the contacts may chatter or bounce relative to each other if the device is subjected to shock or vibration as the stem commences such movement. Also, the stem tends to move slower in moving upwardly than in moving downwardly. If curve 45 had a slope adjacent bottom stop limit 51 similar to the slope of the same curve adjacent top stop limit 52, the operation of the switch in either direction would be essentially identical, and contact chatter or bounce near the bottom stop limit would be minimized or eliminated.

FIG. 8 depicts the operation of switch 10, including movable contact mounting assembly 44, operating through deflection range D, it being assumed that biasing mechanism 34 is regulated to operate through deflection range 53 of curve 45 as shown in FIGS. 6 and 7. In FIG. 8, curve 97 depicts the force applied to stem 22 by mounting assembly 44 as the stem moves between its two stops, and curve 98 depicts the total force applied to the stem by biasing mechanism 34 and mounting assembly 44. That is, in FIG. 8, curve 98 represents the summation of curves 45 and 97 through stem displacement range D. Line 99 is the desired net slope of the summation curve and line 100 is the actual net slope of the summation curve. In FIG. 8, as in the other graphical representations in the accompanying drawings, force F is the force applied upwardly on the stem by the pressure of fluid applied to diaphragm 16, it being understood that the force applied by biasing mechanism 34 throughout stem displacement range D actually is applied downwardly to the stem.

Force/deflection curve 97 of mounting assembly 44, as depicted in FIG. 8, has a negative value 101 at point 102 corresponding to bottom stop limit 51 of stem displacement range D. That is, when the stem is at its lower limit of travel, the force exerted by contact mounting assembly 44 on the stem acts upwardly on the stem, but is negative relative to the force exerted on the stem by biasing mechanism 34.

When the stem is engaged with its bottom displacement stop, switch contacts 25 and 28 (see FIG. 2) are engaged and contact support leaf 90 is disengaged from pick 91 so that the entire length of leaf 90, from contact pad 25 to the adjacent end of support lever 55, is effective to develop force to bias contacts 25 and 28 together. As the stem begins to move upwardly from its bottom stop, the right end of support lever 55 begins to move downwardly (such difference in direction of travel between the stem and the support lever being the result of the geometry of the contact mounting assembly), thereby moving pick 91 toward leaf 90 and causing the force developed in the leaf to decrease essentially linearly until, at point 103 on curve 97, the pick engages leaf 90, contacts 25 and 28 still being engaged. As pick 91 engages leaf 90, the effective length of the leaf is significantly reduced and its effective stiffness is significantly increased as depicted in FIG. 8 by the difference in slope of that portion of curve 97 lying between points 102 and 103 as compared to that portion of curve 97 lying between points 103 and 104. Point 104 represents the location, or instant along the travel of stem 22 from its bottom to its top stop, at which movable switch contact 25 separates from stationary switch contact 28.

At point 105 of curve 97 (FIG. 8) stem 22 has moved upward sufficiently that movable contact 26 engages stationary contact 29. At point 106, the stem has moved sufficiently to cause pick 93 to disengage from leaf 92, and at point 107 the stem has reached its top stop and come to rest. As noted above, leaf 92 is somewhat thicker, and therefore stiffer, than leaf 90. Therefore, curve 97 has greater slope between points 105, 106 and 106, 107 than between points 102, 103 and 103, 104, respectively. When the stem rests against its top limit of travel, mounting assembly exerts a force, having value 100, downwardly on the stem.

It will be understood that a reverse procedure, described by curve 97, occurs as stem 22 moves through displacement range D in moving from its upper limit of travel to its lower limit of travel.

Between points 104 and 105, the mounting assembly is in such position that neither of movable contacts 25 and 26 is engaged with stationary contacts 28 and 29. Therefore, neither of leaves 90 or 92 is effective to exert any force upon stem 22. It will be understood, however, that between points 104 and 105 the mounting assembly does exert some force upon the stem by reason of the flexure of springs '70, 71 and 60. The forces exerted upon the stem by springs 70, 71 and 60 are quite small in relation to the forces exerted by leaves 90 and 92 because of the geometry of assembly 44 which provides a mechanical advantage of about 7:1 to 8:] in terms of the effect on stem 22 of the forces actually developed in the leaves. Moreover, to represent in FIGS. 8-12 the forces applied to the stem by springs 70, 71 and 60 would complicate these graphs unduly and would not significantly contribute to an understanding of the invention. It is sufficient to note that the forces developed by springs 70, 71 and 60 shift curve 97 only but slightly relative to the horizontal (zero force) reference axis in FIG. 8.

The vertical dimension in FIG. 8 is foreshortened to indicate that the magnitudes of the forces associated with'curve 97 are small relative to the magnitudes of the forces associated with force/deflection curve 45 of biasing mechanism 34. The forces associated with curve 97 are only so large as to produce the desired modification of the shape of curve 45 throughout stem displacement range D.

As noted above, curve 98 of FIG. 8 represents the summation of curves 97 and 45, i.e., the summation of the forces associated with curves 97 and 45 throughout stem displacement range D. The net slope of summation curve 98 is represented by line 100 which has a positive slope indicative of a positive spring rate for the combination of mounting assembly 44 and bias mechanism 34 when the bias mechanism is operated through range 53 as stem 22 moves through distance D. It will be recalled, however, that a featured characteristic of switch 10, at least in the preferred embodiment here under consideration, be that the complete mechanism exhibit a negative spring rate through travel of the stem, thereby to produce the desired toggle action associated with a bistable switch. If the device is to be responsive in the desired manner to a pressure induced force differential AF then the desired slope of the summation curve should be as indicated byline 99 of FIG. 0.

A summation curve having a net negative slope described by line 99 can be obtained, using mounting assembly 44 mechanism 34 so that, throughout travel of the stem through distance D, the operation of the biasing mechanism is described by a portion of curve 45 different from that represented by range 53 shown in FIG. 6. Thus, by operating biasing mechanism 34 through that portion of curve 45 represented by range (see FIG. 6) during movement of stem 22 through distance D, and without changing mounting mechanism 44 from the condition thereof productive of curve 97, there is produced a summation curve 111 (see FIG. 9) having a net slope coincident with desired slope 99 when curve 97 and the portion of curve 45 in range 110 are summed. Portion 110 of curve 45 is selected to be that portion of the curve which has a net slope 112 which, when summed with the net slope 113 of curve 97, produces slope 99. As is apparent from a comparison of FIGS. 0 and 9, the stiffness of biasing mechanism 34 may need be increased slightly to produce a device responsive to force differential AF in which the limits of such differential are ofthe desired values.

FIG. 10 is a representation of summation curve 111 (FIG. 9) in which areas 115 and 116 have been shaded in a manner corresponding to areas 95 and 96, respectively, of FIG. 7. Thus, area 115 in FIG. 10 represents the energy which must be expended to move stem 22 from its bottom stop to its top stop, and area 116 represents the energy which must be expended to drive the stem in the reverse direction. It will be noted that area 115 is greater than area 116, and that curve 111 has an excursion below force range AF It is desired that areas 115 and 116 be as nearly equal as possible so that substantially equal energies are associated with stem movements in opposite directions. Excursions of curve 111 out of force range A1 are not desired since such excursions denote reductions in the momentum of the stem in moving in one direction or the other, indicating that operation of the switch in one direction is substantially different from switch operation in the reverse direction. On the favorable side, curve 111 has substantially the same slope between points 117 and 118 thereof as between points 121 and 122, indicating that initial increments of movement of the stem in one direction occur at the same rate as initial increments of movement of the stem in the other direction. The optimum summation curve for the presently preferred switch described herein is a curve which has the general shape of curve 111 throughout movement of stem 22 over displacement D but which is symmetrical, or nearly so, about desired slope line 99; summation curve 123 of FIGS. 11 and 12 represents a more nearly optimum situation in terms of a presently preferred embodiment of the invention.

In FIG. 10, points 117122 of curve 111 correspond to the conditions which exist in switch 10 at the instants associated with points 102-107 of curve 97.

Curve 123 of FIG. 11 is the curve which results from the summation of the force/deflection curve 124 (FIG. 12) of mounting assembly 44, adjusted to produce force/deflection conditions different from those depicted by curve 97, and of force/deflection curve 125 of biasing mechanism 34 operated through a range of curve 45 lying between the opposite limits of ranges 53 and 110 (see FIG. 6). Summation curve 123 has origin and inflection points 126131 corresponding to points 133-138 of curve 124, such points corresponding in significance to points 117122 of curve 111 and to points 102- --107 of curve 97, respectively.

Comparing FIGS. 9 and 12, especially curves 97 and 124 thereof, it will be seen that different curves 97 and 124 can be produced by structure 44 adjusted in the manner described below. Specifically, curve 124 differs from curve 94 in that l) the force 139 exerted on stem 22 by the mounting assembly, adjusted to produce curve 124, is of smaller magnitude than force 101 when the stem is at its lower limit of travel, 2) the force associated with point 134 (curve 124) is less than that associated with point 103 (curve 97), 3) the slope of curve 124 between points 133 and 134 is less than the slope of curve 97 between points 102 and 103, thereby indicating a difference in stiffness of the spring leaves producing such forces, 4) point 136 lies closer to point than does point 105 to point 104 on curve 97, 5) the force associated with point 137 (curve 124) is less than the force associated with point 106 (curve 97), 6) the force 140 exerted by the mounting assembly of curve 124 when the stem is at its upper limit of travel is less than the force 108 exerted by the assembly of curve 97, and 7) the slope of curve 124 between points 137 and 138 is less than the slope of curve 97 between corresponding points 106 and 107, thereby indicating a difference in stiffness of the spring leaves producing the forces associated with points 137, 138 and 106, 107.

The difference of the slope of curve 124 between points 133 and 134 relative to the slope of curve 97 between points 102 and 103 is produced by thinning the sheet material from which leaf 90 is made, thereby slightly reducing the stiffness of this leaf. The magnitude of the force applied to the stem by the mounting assembly in the lowermost position of stem 22 is reduced from force 101 to force 139 by reducing the stiffness of leaf 90 and by adjusting the normal (i.e., undeflected) position of leaf 90 so that the leaf is not deflected as far from its pick when the stem is engaged with its bottom stop.

The lateral shift to the left of point 136 of curve 124 relative to point 105 of curve 97 may be accomplished by moving the tip of pick 93 away from the tip of pick 91, thereby increasing the spacing between contacts 25 and 26 in the direction thereof so that contact 26 engages contact 29 earlier during movement of the stem from its bottom to its top limit of travel.

The distance between points 136 and 137 on curve 124 is reduced relative to the distance between points 105 and 106 on curve 97 by the foregoing adjustment of pick 93 since such adjustment reduces the time, during movement of the stern, during which leaf 92 engages pick 93 after contacts 26 and 29 engage, assuming the distance between contacts 28 and 29 remains constant. This change in distance along corresponding portions of curves 97 and 124 is also accomplished by reducing the thickness, and thereby the stiffness, of leaf 92, which thickness reduction is desired to reduce the magnitude of the force applied to the stem at the uppermost limit of travel thereof from force value 108 to force value 140. The change in thickness of leaf 92 also reduces the slope of curve 124 between points 137 and 138 relative to the slope of the corresponding portion of curve 97.

The net effect of the above-described adjustments to the structure of mounting assembly 44 is to alter the force/deflection curve thereof from the shape of curve 97 to the shape of curve 124 which has a net positive slope represented by line 141 in FIG. 12. Once the nature and net slope of curve 124 is established, the net slope 99 of the desired summation curve 123 being known, it is a simple matter to select that portion of curve 45 which has a net slope 142 sufficient, when added to the net slope of curve 124, to produce slope 99, and to adjust biasing mechanism 34 to operate in the selected range between appropriate units of force value. Thus, one can readily obtain summation curve 123 which has terminal points 126 and 131 defining desired force differential AF Referring again to FIG. 11, it will be seen that curve 123 is such that the energy expended during movement of the stem from its bottom stop to its top stop (such energy being represented by shaded area 144) is substantially equal to the energy (represented by area 145) expended during movement of the stem in the opposite direction. It will also be observed that curve 123 is essentially symmetrical about slope line 99. Thus, the above-described structure productive of curve 123 produces a device having the optimum operating characteristics desired in the switch which constitutes the presently preferred embodiment of the invention described herein.

In FIG. 12, the curve denoted 45 (53) is that portion of curve 45 which lies in range 53, as depicted in FIG. 6, which has the same average slope 99 as curve 123. It will be recalled that curve 45 (53) is the force versus deflection curve which would be used had the teachings of the prior art, rather than the present invention, been followed. FIG. 12, therefore, by comparing curve 123 (the force versus deflection curve of the overall mechanism of switch and curve 45 (53) (the force versus deflection curve of biasing mechanism 34 through a range thereof having an average slope equal to that of curve 123), clearly illustrates the advantages produced by this invention. Of particular significance is the difference between the slopes of curves 123 and 45 (53) adjacent the bottom stop limits thereof.

From the foregoing description, it will be apparent to workers skilled in the art to which the present invention pertains how the structure described, and shown in FIG. 2, may be modified or adjusted to any force/deflection curve different from curves 97 and 124 that may be desired. The structure of FIG. 2, as described or modified, may be used in connection with any suitable biasing mechanism to produce a resultant curve having positive or negative or zero net slope, as desired. The structure of FIG. 2 may be used in a switch mechanism of the type illustrated by switch 10, or in any other switch or other mechanism desired without departing from the scope of this invention.

In using or in modifying the mechanism described above, it should be borne in mind that the stiffness of spring leaves and 92 as manifested at stem 22 is the actual stiffness of such springs multiplied by the square of the mechanical advantage of the assembly interconnecting the springs to the stem. This relation means that mounting assembly 44 can be made very light in weight and can use springs of relatively low stiffness and still provide a significant effect upon the force/deflection characteristic of the complete device, which characteristic, in terms of force levels, is defined primarily by biasing mechanism 34 in the preferred structure described above.

Therefore, as demonstrated above, this invention effectively and efficiently provides a solution to the problems which were associated with prior devices of the type exemplified by switch 10. These problems are solved by structure which is simple and reliable in operation and which is light in weight. The light weight of the present structure means that its presence in switch 10 does not render the switch noticeably more sensitive to acceleration and shock than prior switches, and in this respect the present structure is to be compared with prior solutions to the problems solved by this invention.

Iclaim:

1. A bistable electric switch mechanism comprising:

a housing;

movable contact means in the housing having two stable positions;

stationary contact means in the housing engageable with the movable contact means in each of the stable positions of the movable contact means;

bistable actuating means connected to the movable contact means responsive to actuating stimuli having values greater than a first selected value and less than a second selected value for moving the movable contact means between said stable positions;

biasing means, included in the bistable actuating means,

urging the movable contact means to one of said stable positions and against which an actuating stimulus must act to produce movement of the movable contact means to its other stable position, the biasing means throughout the range between the stable positions of the movable contact means having a nonlinear negative spring rate the value of which is less when the movable contact means is at its one position than when the movable contact means is at its other position;

the improvement comprising resilient means cooperating with the movable contact means and having an effective spring rate and stiffness in the one position of the movable contact means which is different from the effective spring rate and stiffness thereof in the other position of the movable contact means, the resilient means and the biasing means cooperating to define an overall negative spring rate for the switch mechanism and against which an actuating stimulus must act and which has substantially the same value when the movable contact means is at its one position as when the movable contact means is at its other position.

"2. The bistable electric switch mechanism of claim 1 wherein the stationary contact means includes a pair of spaced contact members, the movable contact means includes a pair of contact members one of which is engaged with one of the stationary contact members in one of the stable positions of the movable contact means and the other of which is engaged with the other stationary contact member in the other stable position of the movable contact means.

3. Apparatus according to claim 2 wherein the switch mechanism has a composite force versus deflection characteristic through an operating range of the actuating means between limits defined by the stable positions of the movable contact means, such characteristic when plotted on Cartesian coordinates having a net slope line, and the biasing means and the resilient means are cooperatively selected and arranged to define a composite characteristic which is substantially symmetrical about the net slope line between said limits when plotted on said Cartesian coordinates.

4. Apparatus according to claim 3 wherein the biasing means and the resilient means are cooperatively. selected and arranged so that said composite characteristic has values between said limits neither greater nor lesser than the values thereof at said limits. 7

5. Apparatus according to claim 1 wherein the switch mechanism has a composite force versus deflection characteristic between limits defined by the stable positions of the movable contact means, and the biasing means and the resilient means are cooperatively selected and arranged so that the force. aspect of said characteristic is defined essentially by the biasing means.

6. Apparatus according to claim 5 wherein, when said characteristic is plotted graphically on selected coordinates, the plot thereof between said limits has a configuration determined substantially by the resilient means.

7. Apparatus according to claim 1 wherein the stationary contact means includes a pair of spaced contact members, the

' movable contact means includes a pair of contact members one of which is engaged with one stationary contact member in the one position of the movable contact means and the other of which is engaged with the other stationary contact member in the other position of the movable contact means, and the resilient means includes an essentially rigid lever pivotally mounted tothe housing and connected to the actuating means for movement in response to operation of the actuating means, separate spring means mounting each movable contact member to the lever, the spring means mounting the one movable contact member to the lever having a spring stiffness which is less than the stiffness of the spring means mounting the other movable contact member to the lever.

8. Apparatus according to claim 7 in which the spring means comprises a spring leaf for each movable contact member, each leaf extending in cantilever fashion from the lever to the associated movable contact member.

9. Apparatus according to claim 8 wherein the lever, the means connecting the lever to the actuating means, and the stationary contact means are cooperatively arranged so that the movable contact means are urged to move past the corresponding stationary contact means in response to operation of the actuating means in corresponding directions whereby each movable contact member is forcibly engaged with its stationary contact member, during periods of such engagement, by deflection of the corresponding leaf.

10. Apparatus according to claim 9 including a pick member for each leaf, each pick member being mounted to thelever and arranged to engage the corresponding leaf adjacent the movable contact member on the side of the leaf opposite to the direction the leaf moves in disengagement of the movable contact member from the stationary contact member after the lever has moved a selected distance in response to operation of the actuating means to produce disengagement of the movable contact member carried by the leaf from its corresponding stationary contact member.

11. Apparatus according to claim 10 wherein each pick member is arranged to engage the adjacent leaf, during operation of the actuating means in a manner productive of disengagement of the movable contact member carried by the leaf from its stationary contact member, at a time before the movable contact member disengages from the stationary contact member.

12. Apparatus according to claim 11 in which the movable contact means and the stationary contact members are arranged so that during a portion of the period of operation of the actuating means neither movable contact member is engaged with a stationary contact member.

113. Apparatus according to claim 11 in which the spring leaves extend from a common end of the lever.

M. Apparatus according to claim 11 wherein the movable contact means and the resilient mounting means therefor are arranged so that the force of engagement of the one movable contact member and the one stationary contact member in the one position of the movable contact means acts upon the actuating means in a manner additive to action of actuating stimuli upon the actuating means, and so that the force of engagement of the other movable contact member and the other stationary contact member in the other position of the movable contact means acts upon the actuating means in a manner counter to the action of actuating stimuli upon the actuating means.

15. Apparatus according to claim 14 in which the lever and the means connecting the lever to the actuating means are arranged to magnify at the actuating means forces of engagement between the movable and stationary contact members.

116. A pressure sensitive electrical switch including a. a housing defining an internal cavity,

b. a diaphragm disposed across the cavity with its periphery clamped to the housing,

c. means for applying switch operating pressure to the cavity on one side of the diaphragm,

d. an operating stern connected to the diaphragm for reciprocation along the length thereof in response to deflection of the diaphragm by switch operating pressure,

e. limiting means coupled to the stem defining two spaced limits of reciprocable travel of the stem,

f. biasing means coupled to the stem for urging the stem toward one of the limits of travel thereof, the biasing means having a nonlinear negative spring rate and being arranged so that the stem moves from its one to its other limit when switch operating pressure exceeds a selected pressure P, and the stem moves from its other to its one limit when switch operating pressure is less than a second selected pressure P g. a pair of spaced stationary switch contacts mounted to the housing laterally of the stem,

h. a pair of movable switch contacts one engaged with one stationary contact when the stem is at its one limit of travel and the other engaged with the other stationary contact when the stem is at its other limit of travel, and

. motion amplifying linkage means coupling the movable contacts to the stem so that the movable contacts move opposite to the stem in response to movement of the stem and including 1. a substantially rigid lever mounted for pivotal movement relative to the housing,

2. a spring leaf for each movable contact extending from one end of the lever toward the stationary contacts and mounting one of the movable contacts at its end spaced from the lever, the leaves being arranged so that the movable contacts effectively cooperate between the stationary contacts in response to movement of the stem, each leaf having an undeflected condition such that the movable contact carried thereby tends to be movable past the corresponding stationary contact in response to movement of the stem in a contact engaging direction whereby each leaf is deflected and the movable contact thereof is forcibly engaged with the corresponding stationary contact when the stem is at a corresponding limit of travel thereof, each leaf having an efiective stiffness when the movable contact mounted thereto is engaged with its corresponding stationary contact which is different from the effective stiffness of the other leaf in a like condition, and

means carried by the lever in association with each leaf for engaging the leaf adjacent the movable contact member carried thereby for holding the leaf in a deflected state which is less than the deflection of the leaf when the movable contact carried thereby is engaged with its stationary contact so that the leaf is engaged by said means at a time during travel of the stem when the stem is between said limits,

and in which, when the integrated force versus deflection properties of the biasing means and the linkage means are plotted graphically on Cartesian coordinates (force applied to the stem being plotted vertically and stem displacement from its one limit to its other limit being plotted horizontally) there results a curve having a negative slopethereof at and adjacent the one limit of stem travel which is substantially more equal to the negative slope of said curve at and adjacent the other limit of stem travel than is the slope of the curve of the force versus deflection characteristic of the biasing means along (when plotted on said coordinates) at and adjacent the one limit relative to the slope of the biasing means curve at and adjacent the other limit.

17. Apparatus according to claim 16 wherein the integrated force versus deflection curve of the switch is substantially symmetrical about a straight line between the ends of said curve.

18. Apparatus according to claim 17 in which no portion of said integrated curve intermediate the ends thereof lies outside a force range defined by the ends of said curve.

19. Apparatus according to claim 16 wherein a central portion of the integrated force versus deflection curve is essentially coextensive with a corresponding portion of the biasing means force versus deflection curve.

Po-ww UNITED STATES PATENT oEFIcE 569 CERTIFICATE OF CORRECTION Patent No. 3,576,410 Dated April 27, 1.971

Imjentor(5) Irven H. Culver It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column' 16, line 4, for "along" read "alone".

Signed and sealed this 28th day of September 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Pa 

1. A bistable electric switch mechanism comprising: a housing; movable contact means in the housing having two stable positions; stationary contact means in the housing engageable with the movable contact means in each of the stable positions of the movable contact means; bistable actuating means connected to the movable contact means responsive to actuating stimuli having values greater than a first selected value and less than a second selected value for moving the movable contact means between said stable positions; biasing means, included in the bistable actuating means, urging the movable contact means to one of said stable positions and against which an actuating stimulus must act to produce movement of the movable contact means to its other stable position, the biasing means throughout the range between the stable positions of the movable contact means having a nonlinear negative spring rate the value of which is less when the movable contact means is at its one position than when the movable contact means is at its other position; the improvement comprising resilient means cooperating with the movable contact means and having an effective spring rate and stiffness in the one position of the movable contact means which is different from the effective spring rate and stiffness thereof in the other position of the movable contact means, the resilient means and the biasing means cooperating to define an overall negative spring rate for the switch mechanism and against which an actuating stimulus must act and which has substantially the same value when the movable contact means is at its one position as when the movable contact means is at its other position.
 2. The bistable electric switch mechanism of claim 1 wherein the stationary contact means includes a pair of spaced contact members, the movable contact means includes a pair of contact members one of which is engaged with one of the stationary contact members in one of the stable positions of the movable contact means and the other of which is engaged with the other stationary contact member in the other stable position of the movable contact means.
 2. a spring leaf for each movable contact extending from one end of the lever toward the stationary contacts and mounting one of the movable contacts at its end spaced from the lever, the leaves being arranged so that the movable contacts effectively cooperate between the stationary contacts in response to movement of the stem, each leaf having an undeflected condition such that the movable contact carried thereby tends to be movable past the corresponding stationary contact in response to movement of the stem in a contact engaging direction whereby each leaf is deflected and the movable contact thereof is forcibly engaged with the corresponding stationary contact when the stem is at a corresponding limit of travel thereof, each leaf having an effective stiffness when the movable contact mounted thereto is engaged with its corresponding stationary contact which is different from the effective stiffness of the other leaf in a like condition, and
 3. means carried by the lever in association with each leaf for engaging the leaf adjacent the movable contact member carried thereby for holding the leaf in a deflected state which is less than the deflection of the leaf when the movable contact carried thereby is engaged with its stationary contact so that the leaf is engaged by said means at a time during travel of the stem when the stem is between said limits, and in which, when the integrated force versus deflection properties of the biasing means and the linkage means are plotted graphically on Cartesian coordinates (force applied to the stem being plotted vertically and stem displacement from its one limit to its other limit being plotted horizontally) there results a curve having a negative slope thereof at and adjacent the one limit of stem travel which is substantially more equal to the negative slope of said curve at and adjacent the other limit of stem travel than is the slope of the curve of the force versus deflection characteristic of the biasing means along (when plotted on said coordinates) at and adjacent the one limit relative to the slope of the biasing means curve at and adjacent the other limit.
 3. Apparatus according to claim 2 wherein the switch mechanism has a composite force versus deflection characteristic through an operating range of the actuating means between limits defined by the stable positions of the movable contact means, such characteristic when plotted on Cartesian coordinates having a net slope line, and the biasing means and the resilient means are cooperatively selected and arranged to define a composite characteristic which is substantially symmetrical about the net slope line between said limits when plotted on said Cartesian coordinates.
 4. Apparatus according to claim 3 wherein the biasing means and the resilient means are cooperatively selected and arranged so that said composite characteristic has values between said limits neither greater nor lesser than the values thereof at said limits.
 5. Apparatus according to claim 1 wherein the switch mechanism has a composite force versus deflection characteristic between limits defined by the stable positions of the movable contact means, and the biasing means and the resilient means are cooperatively selected and arranged so that the force aspect of said characteristic is defined essentially by the biasing means.
 6. Apparatus according to claim 5 wherein, when said characteristic is plotted graphically on selected coordinates, the plot thereof between said limits has a configuration determined substantially by the resilient means.
 7. Apparatus according to claim 1 wherein the stationary contact means includes a pair of spaced contact members, the movable contact means includes a pair of contact members one of which is engaged with one stationary contact member in the one position of the movable contact means and the other of which is engaged with the other stationary contact member in the other position of the movable contact means, and the resilient means includes an essentially rigid lever pivotally mounted to the housing and connected to the actuating means for movement in response to operation of the actuating means, separate spring means mounting each movable contact member to the lever, the spring means mounting the one movable contact member to the lever having a spring stiffness which is less than the stiffness of the spring means mounting the other movable contact member to the lever.
 8. Apparatus according to claim 7 in which the spring means comprises a spring leaf for each movable contact member, each leaf extending in cantilever fashion from the lever to the associated movable contact member.
 9. Apparatus according to claim 8 wherein the lever, the means connecting the lever to the actuating means, and the stationary contact means are cooperatively arranged so that the movable contact means are urged to move past the corresponding stationary contact means in response to operation of the actuating means in corresponding directions whereby each movable contact member is forcibly engaged with its stationary contact member, during periods of such engagement, by deflection of the corresponding leaf.
 10. Apparatus according to claim 9 including a pick member for each leaf, each pick member being mounted to the lever and arranged to engAge the corresponding leaf adjacent the movable contact member on the side of the leaf opposite to the direction the leaf moves in disengagement of the movable contact member from the stationary contact member after the lever has moved a selected distance in response to operation of the actuating means to produce disengagement of the movable contact member carried by the leaf from its corresponding stationary contact member.
 11. Apparatus according to claim 10 wherein each pick member is arranged to engage the adjacent leaf, during operation of the actuating means in a manner productive of disengagement of the movable contact member carried by the leaf from its stationary contact member, at a time before the movable contact member disengages from the stationary contact member.
 12. Apparatus according to claim 11 in which the movable contact means and the stationary contact members are arranged so that during a portion of the period of operation of the actuating means neither movable contact member is engaged with a stationary contact member.
 13. Apparatus according to claim 11 in which the spring leaves extend from a common end of the lever.
 14. Apparatus according to claim 11 wherein the movable contact means and the resilient mounting means therefor are arranged so that the force of engagement of the one movable contact member and the one stationary contact member in the one position of the movable contact means acts upon the actuating means in a manner additive to action of actuating stimuli upon the actuating means, and so that the force of engagement of the other movable contact member and the other stationary contact member in the other position of the movable contact means acts upon the actuating means in a manner counter to the action of actuating stimuli upon the actuating means.
 15. Apparatus according to claim 14 in which the lever and the means connecting the lever to the actuating means are arranged to magnify at the actuating means forces of engagement between the movable and stationary contact members.
 16. A pressure sensitive electrical switch including a. a housing defining an internal cavity, b. a diaphragm disposed across the cavity with its periphery clamped to the housing, c. means for applying switch operating pressure to the cavity on one side of the diaphragm, d. an operating stem connected to the diaphragm for reciprocation along the length thereof in response to deflection of the diaphragm by switch operating pressure, e. limiting means coupled to the stem defining two spaced limits of reciprocable travel of the stem, f. biasing means coupled to the stem for urging the stem toward one of the limits of travel thereof, the biasing means having a nonlinear negative spring rate and being arranged so that the stem moves from its one to its other limit when switch operating pressure exceeds a selected pressure P1 and the stem moves from its other to its one limit when switch operating pressure is less than a second selected pressure P2, g. a pair of spaced stationary switch contacts mounted to the housing laterally of the stem, h. a pair of movable switch contacts one engaged with one stationary contact when the stem is at its one limit of travel and the other engaged with the other stationary contact when the stem is at its other limit of travel, and i. motion amplifying linkage means coupling the movable contacts to the stem so that the movable contacts move opposite to the stem in response to movement of the stem and including
 17. Apparatus according to claim 16 wherein the integrated force versus deflection curve of the switch is substantially symmetrical about a straight line between the ends of said curve.
 18. Apparatus according to claim 17 in which no portion of said integrated curve intermediate the ends thereof lies outside a force range defined by the ends of said curve.
 19. Apparatus according to claim 16 wherein a central portion of the integrated force versus deflection curve is essentially coextensive with a corresponding portion of the biasing means force versus deflection curve. 